Belt-fed machine guns are often referred to as “crew-served,” because they are too heavy to be practically carried and operated by a single soldier. Such arms have important uses in the battlefield that justify their weight and the commitment of manpower to field them. However, even when mounted on a vehicle, aircraft, or naval craft, the weight of a machine gun is often a disadvantage that it is desirable to minimize. Efforts to reduce weight by substituting unconventional materials (e.g. titanium, aluminum, polymer) for conventional proven steel firearms have disadvantages (e.g. cost, galling, durability.)
Existing belt fed machine guns (e.g. M2, M1919, M240) are typically formed with a receiver in the form of a steel box in which a bolt reciprocates, with a barrel extending from the receiver, and a belt passage extending transversely though the receiver. The box is formed of side plates having guides (slots or ribs) that guide the bolt movement, a bottom plate, and a top plate that opens to facilitate loading. A cylindrical trunnion or barrel-receiving collar is attached at a forward end of the box, and a rear plate is attached at the rear end of the box, with a buttstock or handle attached. The internal bolt guides, external guides for an external charging handle, and any other reinforcing portions are formed as extra sheets of metal, which are overlaid on the main sheets, and riveted in place.
Conventional box-like receivers are assembled from separate parts, with overlapping plate edges that are riveted together for adequate strength. Side plates are flat sheets, and a u-shaped bottom plate has upturned edges that overlap the bottom edges of the side plate for attachments by rivets that pass through both pieces. Where added strength or wear resistance is needed (at the edges that define a charging-handle slot, for instance), strength elements such as metal strips of L-bars are riveted in place. Such an assembly technique creates added weight, due to the necessary overlapping of the riveted parts. Where strength elements are added, or any overlap is needed for riveting, the size of the part or overlap may need to be increased to provide an adequate radius around each rivet, further increasing the part size and firearm weight.
Moreover, the conventional fabrication from plates or sheet metal requires an essentially constant thickness for several reasons. These include manufacturing cost constraints, the need for an enclosed receiver to prevent incursion of dirt, and concerns regarding warpage when sheets are machined.
Seams, rivet holes, and other points of overlap may create opportunities for corrosion. Even with corrosion-resistant coatings, where sheets overlap for attachment, a small gap exists in which moisture can wick, and in which rust generally develops over time, reducing the useful life of the firearm.
Fabrication of receivers from multiple assembled components has further disadvantages. Numerous parts are required to be inventoried for manufacturing and repair. Dimensional errors may accumulate due to imprecision of assembling the components. With the effects of heat-treating and coatings, dimensions may shift due to warpage. The intense forces during operation may further loosen these connections, making a firearm unreliable or dangerous. Larger tolerances that are required to compensate for these issues may reduce potential accuracy of the firearm.
The present invention overcomes the limitations of the prior art by providing a firearm having a body formed of a unitary first shell and a unitary second shell. Each shell defines an open cavity and has a peripheral rim. The rims of the shells are connected together so that the cavities define a receiver chamber. A bolt reciprocates within the receiver chamber, and a barrel connected to the shells has a chamber positioned for operable engagement by the bolt.
The shells may together define a gas tube, and may define a barrel receiving element, each shell having a barrel engagement element. Each shell may have a side panel with flat portions having a limited thickness, and elongated ribs of a greater thickness. The ribs may extend about the periphery of planar side panels, and may extend between the periphery and selected stress elements on the body. A bolt handle slot may have opposed rows of spaced apart bolt handle guides in an alternating arrangement.